1. Field of the Invention
The present invention pertains to fluid pressure sensing and indicating devices. In particular, it pertains to pressure apparatus particularly suitable for sensing and indicating pressures of mud systems, accumulators, wellheads, pipelines and related oil and/or gas equipment.
2. Description of the Prior Art
Apparatus for sensing and indicating fluid pressures are well known. Such devices are used in a wide variety of applications. There are many types of fluid pressure indicating devices, the type used being primarily dependent upon the particular application.
The bourdon tube pressure gauge is commonly used in many applications. A gauge of this type includes a flattened tube of spring bronze or steel formed in a circular pattern and one end of which is non-attached. When pressure is applied to the tube, the tube tends to straighten, actuating a mechanism which is usually provided with a pointer and registration with adjacent indicia of which indicates the pressure applied to the gauge. Because such gauges use hollow tubes, they may be easily plugged by heavy fluid or fluids containing debris or particulate matter. They are also affected by shock and vibration and not too well suited for systems in which large pressure surges may occur.
For the above-stated reasons, the bourdon tube type gauge is not suitable for many petroleum industry applications. This is particularly true of mud systems in which very viscous fluids and fluids containing large amounts of particulates are used in drilling or controlling oil and gas wells. Thus, devices have been developed for use with bourdon tube type gauges in an attempt to adapt such gauges to this application. For example, a protective device is disclosed in U.S. Pat. No. 3,415,123, which isolates the bourdon tube gauge from the mud system yet transmits the pressure thereof to the gauge. Such a device is effective in preventing the gauge from being plugged by the mud. However, it may not prevent the effects of shock vibration and pressure surging. In addition, it increases the complexity of the pressure indicating system, resulting in greater installation, operation and maintenance costs.
Other pressure indicating devices have been developed which totally depart from the bourdon tube type gauge. One such development is the diaphragm gauge which utilizes a fluid filled cavity between two diaphragms. Pressure from the mud system is applied to one diaphragm, causing it to deflect and because of the incompressibility of the fluid in the cavity, results in a corresponding deflection of the other diaphragm. The second diaphragm which is not in contact with the fluid whose pressure is being measured is usually in contact with some type of translating mechanism which translates the deflection of the diaphragm to movement of an indicator needle which in cooperation with a calibrated dial indicates the pressure of the mud system. Examples of such devices may be seen in U.S. Pat. Nos. 2,297,679 and 4,166,396, widely accepted mud pressure gauges.
Another mud gauge which is similar to diaphragm type gauges is the expanding bellows type shown in U.S. Pat. No. 2,537,729. Although it uses a bellows instead of two diaphragms, the operation of such a gauge is very similar to the diaphragm type gauges mentioned above.
Although diaphragm type gauges are definitely more suited for use with mud systems, they are not totally without problems. One major problem is that the pressure-responsive element of such gauges may be made of a stack of metal gaskets and discs which are pressed together to form a fluid filled cavity. Thus a number of metal to metal sealing surfaces are required to remain leak-tight at pressures of up to 20,000 p.s.i. for a period of time. Many such gauges cannot function leak-tight even for the warranty period. In additon, such gauges are commonly provided with a rubber disc in its pressure-responsive element, which creates friction and causes errors in measurement. Such gauges are also complex, utilizing a large number of parts susceptible to manufacturing errors and very expensive to manufacture and maintain. Thus, even though the diaphragm type pressure gauge has been widely used over the past few decades, it leaves much to be desired. However, because it has been the only type which comes anywhere close to meeting the needs, it continues to be used.